Bowing to the Pentagon, the Senate agreed after impassioned debate Thursday to leave the authority to prosecute rapes and other serious crimes with U.S. military commanders in a struggle that highlighted the growing role of women in Congress.

The vote was 55-45 in favour of stripping commanders of that authority, but that was short of the 60 necessary to move ahead on the legislation sponsored by Sen. Kirsten Gillibrand. It would have given the decision to take serious crimes to courts-martial to seasoned military trial lawyers, independent of the chain of command.

The debate and vote was the culmination of a nearly yearlong campaign to curb sexual assault in the ranks, led by female senators who have questioned whether the military's mostly male leadership understands differences between relatively minor sexual offences and serious crimes that deserve swift and decisive justice.

Pentagon leaders vigorously opposed the measure, as did former prosecutors and military veterans in the Senate who argued that commanders should have more responsibility, not less, for the conduct of the men and women they lead.

"We can't let the commanders walk away," insisted Sen. Claire McCaskill, who bemoaned the tenor of a policy debate that pitted her against fellow Democrat Gillibrand.

Backers of the measure insisted that piecemeal reforms have had only a limited impact on a problem that even the military has called an epidemic. Survey results have suggested that some 26,000 people, mostly women may have been sexually assaulted in the most recent accounting with thousands unwilling to come forward for fear of inaction or retribution.

Gillibrand's effort divided the Senate in ways that smashed conventional lines on both gender and political party.

Conservative Sens. Ted Cruz and Rand Paul, both possible Republican presidential candidates in 2016, backed her effort, while the chairman of the Armed Services Committee, Democratic Sen. Carl Levin, opposed it. Including Gillibrand, the bill had the support of 17 of the Senate's 20 women.

The rejection was unlikely to be the final word on the issue. Gillibrand is expected to pursue it this spring when the Armed Services Committee begins work on a sweeping defence policy bill for the 2015 fiscal year.

In two hours of debate, proponents and opponents argued on the Senate floor based on personal experiences, growing frustration with what they dismissed as fixes around the edges and horrific stories from the ranks.

"The current system is failing the men and women in uniform," said one of the Senate's newest members, John Walsh, a Democrat who is the first Iraq War veteran in the body. "We have moved too slowly."

On the other side was Sen. Jack Reed, a Democrat who served in the 82nd Airborne Division as an infantry platoon leader and company commander. Reed said stripping commanders of the authority to discipline the troops would be "detrimental to the effectiveness of the force and common goal to reduce sexual assault."

After blocking Gillibrand's bill, the Senate moved toward overwhelmingly passage of a measure sponsored by McCaskill and two Republican senators — Kelly Ayotte and Deb Fischer. That bill would eliminate the "good solider defence" — that a service member's character and military performance can be used in a case — unless it is directly connected to the allegation. And it would allow sexual assault victims to challenge their discharges or separation from service.

The bill also calls for a civilian service secretary review if a prosecutor and commander disagree over whether to litigate a case.

The Senate voted 100-0 to move ahead on that bill. A vote on passage is scheduled for Monday.

The Pentagon came under pressure last month to disclose more information about how sexual assault cases are adjudicated following an Associated Press investigation that found a pattern of inconsistent judgments and light penalties for sexual assaults at U.S. bases in Japan.

The AP's investigation, which was based on hundreds of internal military documents, found that what appeared to be strong cases were often reduced to lesser charges. Suspects were unlikely to serve time even when military authorities agreed a crime had been committed. In two rape cases, commanders overruled recommendations to court-martial the accused and dropped the charges instead.

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